This invention is concerned with a method for obtaining substantially pure orthophosphoric acid from superphosphoric acid.
ORTHOPHOSPHORIC ACID, H.sub.3 PO.sub.4, has found use in a wide variety of industrial and commercial applications such as for the production of phosphates in soaps and detergents, for pickling and rust-proofing metals, in pharmaceuticals and in beverages to name a few. orthophosphoric acid is prepared commercially by two different methods; the thermal method and the wet-process method.
In the thermal method, phosphate rocks are smelted in a furnace at high temperatures to produce phosphorus vapor which escapes from the furnace as a gas. The gas is condensed to produce elemental phosphorus which is then burned with dry air and absorbed in water to produce orthophosphoric acid.
In the wet process method, a phosphorus-containing material is treated with a mineral acid such as sulfuric acid to liberate orthophosphoric acid which is then filtered to remove the calcium salt (gypsum), clarified and concentrated as required by the particular industry.
Superphosphoric acid is formed upon concentration of orthophosphoric acid. Superphosphoric acid is a liquid containing a mixture of orthophosphoric acid and polyphosphoric acids such as pyrophosphoric, tripolyphosphoric acid and higher poly acids which on dilution with water hydrolyze to orthophosphoric acid. Superphosphoric is used for manufacturing liquid fertilizers where the polyphosphates act as a sequestering agent for metallic impurities. Generally, superphosphoric acid is classified on the basis of its P.sub.2 O.sub.5 content. For example, a commercial superphosphoric acid may contain 69-72% P.sub.2 O.sub.5 and about 20-60% poly P.sub.2 O.sub.5. The percent P.sub.2 O.sub.5 refers to the total P.sub.2 O.sub.5 content of the superphosphoric acid while the percent poly P.sub.2 O.sub.5 refers to the percent of the total P.sub.2 O.sub.5 in the polyphosphoric acid form. As used herein, the term superphosphoric acid applies to any mixture of orthophosphoric acid and polyphosphoric acid or acids containing at least 62% P.sub.2 O.sub.5 and 1% poly P.sub.2 O.sub.5.
It is important in many applications to employ substantially pure orthophosphoric acid. Unfortunately in the wet-process method, phosphoric acid is not obtained in a pure state and contains significant amounts of inorganic impurities such as calcium phosphate, calcium sulfate and compounds of iron, aluminum, chromium, vanadium and magnesium. Among the prior art processes used for purifying phosphoric acid obtained by the wet-process method include organic solvent extraction, impurity precipitation, ion-exchange crystallization.
Both the solvent extraction and impurity precipitation methods produce significant amounts of by-product phosphate as impure acid or salts which must be disposed of. Solvent extraction may also leave carbonaceous residues which discolor the product acid and can be dangerous to use. Moreover, the product must be concentrated by evaporation of water which involves costly heat energy expenditure. Ion-exchange methods also require product evaporation and their commercial usage is questionable. Crystallization of phosphoric acid has previously required concentration of dilute phosphoric acid to about 68-70% P.sub.2 O.sub.5, while insuring that polyphosphoric acid species are not formed. For this reason industrial use of the crystallization method has not materialized.
The instant invention, provides an economical and efficient method for obtaining substantially pure orthophosphoric acid which method avoids the problems of water evaporation and concentration and presents no safety problems as in solvent extraction.